This invention pertains to membranes used for preferentially concentrating one or more gases from a mixture of gases. More particularly, the invention is applicable to anisotropic membranes, sometimes referred to as composite, or asymmetric membranes. The invention especially concerns a treatment process for controlling the microporosity of the thin, dense layer or composite coating of anisotropic membranes. This layer or coating provides selectivity in separations employing such membranes.
Selective separation to upgrade the concentration of one or more gases in a gaseous mixture is especially important in chemical processing, tertiary oil recovery, and gas purification processes. To be commercially attractive in such processes, membrane systems should be capable of withstanding typical separation operating conditions; they should provide an adequate separation of one or more of the gases in the mixture; and they should exhibit a sufficiently high flux (permeation rate per unit area).
The parameters of interest for separation membranes include selectivity and flux. The greater the selectivity, the greater the degree of separation which may be accomplished using the membrane. The higher the flux, the greater the quantity of gas permeate which may be obtained per unit time. In the past, most membranes which have been highly selective have had dense or compact structures which result in undesirably low fluxes. Correspondingly, those membranes which have exhibited high flux rates have had poor selectivity. It has, therefore, been a continuing objective in the art to develop improved membranes which will adequately separate one or more desired gases in a mixture while at the same time provide a sufficiently high flux to be commercially and economically attractive.
Early work with membranes for liquid separations centered around dense or compact, pore-free membranes with no direct fluid communication between surfaces of the membrane. Separation across such membranes obtains by solvent diffusion through the membrane structure. Such diffusive membranes offer high selectivity; however, their rates of diffusion are so low that they have limited utility. It was has been recognized that if such diffusive membranes could be fabricated in extremely thin form, the diffusion path could be short enough to provide acceptable flux rates. To achieve such flux rates, however, the diffusion path must be reduced to less than about 10.sup.-4 cm.
As the thickness of a membrane is reduced its mechanical strength is also reduced. Ultrathin membranes, theoretically possessing attractive flux rates, generally do not have the mechanical strength necessary to withstand the differential pressures necessary to realize those flux rates. This problem was addressed some years ago by composite membranes, also called anisotropic or asymmetric membranes. Loeb et al., U.S. Pat. No. 3,133,132, for example, describes a procedure for preparing a cellulose acetate composite membrane which consists of an ultrathin, microporous layer supported on a significantly more porous substrate. In time, this technology was extended to membranes formed from acrylonitrile, polysulfone, and polyvinyl chloride (see, for example, U.S. Pat. Nos. 3,526,588 and 3,615,024). The supporting substrate of such composite structures is formed with as high a porosity as possible in order to reduce the hydraulic resistance of the membrane. Ideally, the porosity of the support layer will be such that its contribution to the total resistance of the membrane system is negligible compared to that of the ultrathin separation layer.
A problem frequently encountered in separation operations with anisotropic membranes is the presence or formation of "pin holes"--pores within the ultrathin separation layer of the composite membrane. One operational approach to minimizing this problem in liquid solute/solvent separations has been to add a soluble polymer (e.g., a polyacrylic acid) to the recirculating feed. It is thought that the soluble polymer plugs the pin holes and thereby maintains the performance of the membrane.
For gas separations, membrane defects are a far greater problem. The presence of small pores in a membrane used for a liquid phase separation may not significantly affect its performance due to the much greater viscosity of liquids as compared to gases, and also due to such phenomena as solvent absorption and swelling of the membrane. These properties, however, differ by orders of magnitude in the case of gases. Since gases will readily pass through pores in a membrane, little, if any, separation will result if the membrane contains defects. Because gases generally have lower solubilities in membranes than liquids, an adequate flux can be achieved only with the thinnest possible separation membranes. The problems inherent in preparing defect-free ultrathin membranes increase exponentially as their thickness is reduced.
Chemical treatments with solvents or swelling agents and/or annealing have been practiced to reduce pores in gas separation membranes. In these procedures, one surface of a membrane is treated with a solvent or swelling agent to increase the density of the surface layer and thereby reduce the number of pores. The decrease in porosity increases the selectivity; however, the procedure usually results in a substantial decrease in flux.
Several techniques have also been suggested for sealing or plugging pin holes in composite membranes. Browall, U.S. Pat. No. 3,980,456, describes a composite membrane in which breaches or pin holes are sealed by applying a preformed organopolysiloxanepolycarbonate. Henis et al., U.S. Pat. No. 4,230,463, describes a process for coating an anisotropic membrane to plug any pores in the ultrathin dense layer.
Coating of the dense layer, however, may adversely affect total separation. The separation factor of a laminated membrane for any type of permeating molecule will lie somewhere between the maximum and minimum separation factors of the component membranes. (C. H. Lee, Permeation Properties in Laminated Membranes, Separation Science, 9, pp. 479-485, 1974) The effect of lamination on total membrane performance becomes more critical if one of the components changes permeating properties with gas concentration. The Henis et al. patent, mentioned earlier, attempts to circumvent such problems by selecting a rubber or liquid coating material with a transport rate greater by an order of magnitude than the polymer in the base membranes. Although this approach may reduce certain adverse effects of coating, it restricts the selection of coating polymers. Moreover, because of the physical state of such polymers (above their glass transition temperature), their long-term utility in processes requiring high temperatures and pressures is questionable.